This application is based upon and claims the benefit of priority of Japanese Patent Applications No. 2002-32549 filed on Feb. 8, 2002 and No. 2002-372635 filed on Dec. 24, 2002, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an ignition coil for an internal combustion engine (hereinafter called an ignition coil).
2. Description of Related Art
In the past, high voltage was applied to spark plugs via a high tension coil from a mechanical distributor. However, it has been a recent tendency to apply the high voltage directly to each of the spark plugs from an independent type ignition coil provided individually for each cylinder of an internal combustion engine (engine), as disclosed in JP-A-63-70508.
In the independent type ignition coil, an inside of its case (housing) is filled with resin insulating material such as epoxy resin for not only securing better electric insulation between component parts constituting the ignition coil but also holding stably the component parts.
Since the independent type ignition coil is installed in a plug hole of the engine and is likely influenced by heat or vibration from the engine, the resin insulating material in the ignition coil is apt to crack under the influence of thermal stresses due to cooling and heating cycles. It is a problem that a crack in the resin insulating material results in a shortened insulation distance, which can cause insulation breakdown.
To prevent the resin insulating material from cracking, in the conventional ignition coil, a separation tape whose bonding force to the resin insulating material is weak or resin resilient layer is used at the outer circumference of a primary spool to relieve the thermal stresses acting on the resin insulating material, as disclosed in JP-A-10-241974.
However, the use of the separation tape or the resin resilient layer results in increasing the number of component parts and the time necessary for their assembly so that the ignition coil is more expensive.
To achieve an inexpensive ignition coil, it is proposed with a prior Japanese Patent Application No. 2002-144902 filed on May 20, 2002 by the same applicant that the primary spool (outer spool) is made of resin material easily separable from the resin insulating material. However, the present inventors"" experimental test result and analysis reveals a drawback in that a crack tends to occur in the resin insulating material at a position where an axial leading end of the primary spool exists. This is because the resin insulating material, whose bonding strength to the primary spool is weak, is separated from the primary spool by thermal stress due to cooling and heating cycles, which causes steps at an edge corner portion of the resin insulating material at a position opposed to the axial leading end of the primary spool. On the other hand, in the conventional ignition coil the resin material used for the primary spool has strong bonding strength to the resin insulating material and the separation tape is used only at a position where a primary coil is wound on the primary spool, the resin insulating material is firmly adhered to and not separated from the axial leading end of the primary spool.
The present invention has been made as a result of the present inventors"" experimental test, which reveals that a crack is likely to occur on the edge corner portion of the resin insulating material if the axial leading end of the primary coil is at a certain position between a center core and a high voltage metal fitting.
An object of the present invention is to provide an ignition coil for an internal combustion engine in which cracks hardly occur on resin insulating material opposed to an axial leading end of an outer spool by thermal stress due to cooling and heating cycles, even if the outer spool is made of resin material whose bonding strength to the resin insulating material is weak.
To achieve the above object, the ignition coil to be directly connected with a plug terminal of a spark plug has an inner coil unit, outer coil unit, a secondary terminal, a coil case accommodating a substantial part of the inner and outer coil units, a high voltage tower case having a pipe shaped tower case to be coupled with the spark plug and a metal fitting arranged centrally inside the pipe shaped tower case for connecting in circuit the secondary terminal with the plug terminal and a resin insulating material.
The inner coil unit is composed of an inner spool, an inner coil wound on the inner spool and a center core made of magnetic material and housed centrally inside the inner spool. The outer coil unit is composed of an outer spool positioned radially outside the inner coil unit and an outer coil wound on the outer spool. The inner and outer coil units are arranged concentrically. High voltage induced in one of the inner and outer coils is applied to the secondary terminal when the other of the inner and outer coils is energized. The pipe shaped tower case on a side axially opposite to the spark plug is connected with an axial end of the coil case and the metal fitting is provided with a main body that blocks an opening of the pipe shaped tower case to the coil case so that the coil case and the pipe shaped tower case form an inner space which accommodates axial leading ends of the inner and outer spools on a side of the spark plug. The inner space is filled with the resin insulating material.
With the ignition coil mentioned above, the outer spool is made of resin material whose bonding strength to the resin insulating material is weak and the axial leading end of the outer spool is positioned axially away from a reference position by a distance equal to or shorter than 60% of a reference length or by a distance equal to or longer than 90% of the reference length, where the reference position is an axial end of the center core on a side of the spark plug and the reference length is an axial length between the reference position and an axial end of the main body of the metal fitting on a side opposite to the spark plug.
It is preferable that the inner spool is provided with a ring shaped inner flange protruding radially outward for holding an axial end of the inner coil on a side of the spark plug and the axial leading end of the outer spool is positioned axially away from the reference position by a distance same as or longer than a surface of the inner flange that holds the axial end of the inner coil. This construction serves to prevent creeping discharge or short circuit between the inner and outer coils.
Preferably, the outer spool is provided with a ring shaped outer flange protruding radially outward for holding an axial end of the outer coil on a side of the spark plug and a cylindrical outer skirt extending from the outer flange toward the spark plug so that the axial leading end of the outer spool is an axial end of the cylindrical outer skirt on a side of the spark plug.
The outer flange may be at a position axially same as the inner flange, may be positioned on a side axially opposite to the spark plug with respect to the inner flange, or may be positioned on a side of the spark plug with respect to the inner flange.
When the outer flange is positioned on a side axially opposite to the spark plug with respect to the inner flange, it is preferable that the axial end of the cylindrical outer skirt on a side of the spark plug is at a position axially same as or more away from the reference position than a surface of the inner flange that holds the axial end of the inner coil.
The outer spool may have the ring shaped outer flange without the cylindrical outer skirt extending from the outer flange toward the spark plug so that the axial leading end of the outer spool is a surface of the outer flange on a side of the spark plug. In this case, it is preferable that the surface of the outer flange on a side of the spark plug is at a position more away from the reference position than a surface of the inner flange that holds the axial end of the inner coil.